The invention relates to an electrolysis apparatus for producing halogen gases from aqueous alkali halide solution, having a number of plate-like electrolysis cells which are arranged beside one another in a stack and are in electrical contact and which each have a housing comprising two half-shells of electrically conductive material with external contact strips on at least one housing rear wall, the housing having devices for feeding the electrolysis current and the electrolysis starting materials and devices for discharging the electrolysis current and the electrolysis products, and in each case having two essentially flat electrodes (anode and cathode), the anode and cathode being provided with apertures like venetian blinds for the electrolysis starting materials and the electrolysis products to flow through, being separated from one another by a dividing wall and arranged parallel to one another and being electrically conductively connected to the respective associated rear wall of the housing by means of metal reinforcements.
The individual electrolysis cells are produced in such a way that the respective housings are assembled from two half-shells in each case with the interposition of the necessary devices and the cathode and anode as well as the dividing wall, and by fixing the same by means of metal reinforcements, and that the anode and housing and cathode and housing are, respectively, electrically conductively fixed to each other, the plate-like electrolysis cells produced in this way then being arranged electrically conductively beside one another in a stack and braced against one another in the stack for the purpose of providing a lasting contact.
The electrolysis current is fed to the cell stack at one outer cell of the stack, it passes through the cell stack in the essentially vertical direction to the central planes of the plate-like electrolysis cells, and it is discharged at the other outer cell of the stack. As referred to the central plane, the electrolysis current reaches average density values of at least 4 kA/m2.
Such an electrolysis apparatus is disclosed by DE 196 41 125 A1 from the same applicant. In the case of this known electrolysis apparatus, the anode and the cathode are connected to the respective rear wall of the halves of the housing via vertical, web-like metal reinforcements. On the rear side of the anode and cathode half-shells, a vertical contact strip for the electrical contact to the adjacent, identically constructed electrolysis cell is fitted in each case. The current flows via the contact strip through the rear wall into the vertical, web-like metal reinforcements and, from there, starting from the metal contact points, (reinforcement/anode), it is distributed over the anode. After the current has passed through the dividing wall (the membrane), it is picked up by the cathode, in order to flow via the vertical, web-like reinforcements into the rear wall on the cathode side and again into the contact strip and, from there, to enter the next electrolysis cell. In this case, the connection between the current-carrying components is performed by welding. At the weld points, the electrolysis current forms peak current densities.
The vertical, web-like metal reinforcements are designed as webs which are aligned with the contact strips and whose lateral edges, over the entire height of the rear wall and the anode or cathode, rest on the rear wall and the anode or cathode.
The vertical webs subdivide the electrode rear space within the respective half of the housing into individual electrolyte-carrying segments. In order that a completely non uniform distribution of the concentration in the electrolyte along the depth of the respective half of the housing does not occur, in each half of the housing, at the bottom, an inlet distributor is provided, via which the electrolysis starting materials can be fed into the individual segments formed by the webs in the half-shells.
By means of an electrolyzer configured in this way, gas-producing electrolysis processes, such as chloralkali electrolysis, hydrochloric acid electrolysis or alkaline water electrolysis, are carried out. In chloralkali electrolysis, aqueous alkali halide solutions, for example sodium and potassium chloride, are decomposed in the electrolysis cell, under the influence of the electrical current, into an aqueous alkali hydroxide solution, for example sodium or potassium hydroxide solution, and into a halogen gas, for example chlorine and hydrogen. In the electrolysis of water, water is decomposed and hydrogen and oxygen are formed at the electrodes.
The physical separation of the electrode spaces is carried out by means of the dividing wall mentioned at the beginning, in general a diaphragm or a so-called ion exchange membrane. The diaphragm consists of a porous material which is chemically, thermally and mechanically stable with respect to the media, temperatures and pressures occurring in the cell. The ion exchange membrane is generally a perfluorated hydrocarbon. These membranes are gas-tight and virtually liquid-tight, but permit the transport of ions in the electrical field.
A particular characteristic of this electrolysis process is the fact that the diaphragm or the ion exchange membrane is pressed against at least one of the two electrodes. This is necessary since, as a result, the dividing wall is fixed and is therefore largely unloaded mechanically. It is often the case that the dividing wall may rest only on one of the two electrodes, since only in this way can the longest possible service life of all the components (electrodes and dividing wall) be achieved. In the event of direct contact between the dividing wall and both electrodes, in some cases a chemical reaction may take place between the dividing wall and the electrodes or the gases developed at the electrodes. For example, a distance between the membrane and the cathode is established in chloralkali electrolysis, since otherwise the electrolysis catalyst or, in the case of inactivated nickel cathodes, nickel is dissolved out of the electrode. Another example is provided by nickel oxide diaphragms, which are used in alkaline water electrolysis. If there is too small a distance from the hydrogen-developing electrode, the nickel oxide is reduced to nickel and therefore becomes conductive, which eventually leads to a short circuit.
Contact between the membrane or diaphragm and at least one electrode, in the case of gas-developing processes, leads to a build-up of gas in the electrolyte boundary layer between the electrode and the membrane or the diaphragm. This even affects the electrodes mentioned at the beginning, which are configured in such a way that the electrolysis starting materials and the electrolysis products can flow through them. Such electrodes are preferably provided with apertures (perforated sheet metal, expanded metal, woven mesh or thin metal sheets with apertures like venetian blinds), so that in spite of their flat arrangement in the electrolysis cell, the gases formed in the boundary layer during the electrolysis can more easily enter the rear space of the electrolysis cell.
The gas bubbles rising in the electrolyte agglomerate in particular in the edges or borders of the apertures, which edges or borders are oriented downward in the cell, and remain there in the interstices between the contacting dividing wall (membrane) and the edges of the apertures. These bubbles disrupt the transport flow, that is to say the transport of materials through the dividing wall, since they block the membrane exchange surface and therefore make it impassable, that is to say inactive.
In the case of an electrode configuration which has been provided by the applicant to reduce this build-up of gas and which is described in German patent specification DE 44 15 146 C2, the electrodes are profiled by being provided with grooves and holes, for example. In this way, firstly the gas can escape more easily and secondly fresh electrolyte can get into the electrolytically active boundary layer between the electrode and the membrane again. If electrodes profiled in this way are loaded with current densities above 4 kA/m2, the development of gas increases still further, however, and the profiled electrode then reaches limit of its ability to discharge gas.
In addition, in gas-developing electrolysis reactions, it also occurs, such as in the anodic development of chlorine in chloralkali electrolysis or the anodic development of oxygen in alkaline water electrolysis, that there is a separation problem, that is to say the developed gas is not separated from the electrolyte, which leads to the formation of foam. This problem leads to the current density distribution being nonhomogeneous, in particular at current densities above 4 kA/m2. As a result, firstly the service life of the active cell components, such as membranes, diaphragms and electrode activators, is restricted. Secondly, the electrolyzers are consequently also limited with regard to the maximum current density to about 4 kA/m2. Additionally, the formation of foam leads to pressure fluctuations within the electrochemical cell, since the foam at least briefly closes the cell outlet for the gas formed. The outlet is blown free again by means of a slight increase in pressure within the cell, which leads to the known effect of surge flow and to the aforementioned pressure fluctuations. This is disadvantageous for the operation of an electrolyzer.
In addition, the service life, in particular of membranes, is influenced by the concentration distribution. The more homogeneous, for example, the sodium chloride concentration in the anode space of a chloralkali electrolyzer, the greater is the service life of the membrane. In order to achieve a homogeneous electrolyte distribution, either additional circulation is produced via pumps arranged externally, or internal circulation is brought about on the basis of a difference in density by installing a guide plate in the cell.
The object of the invention is to provide an electrolysis apparatus which can be operated even at current densities above 4 kA/m2 and consequently increased production of gas in the boundary layer while maintaining lasting service lives of the membrane and with few pulsations.
According to the invention, with an electrolysis apparatus of the type described at the beginning, this object is achieved by the venetian-blind apertures of the anode and cathode being arranged to be inclined with respect to the horizontal.
By means of this configuration according to the invention, as has been shown, the discharge of gas from the electrolyte boundary layer close to the membrane can be improved in such a way that, for the first time, current densities of from 6 to 8 kA/m2 are achieved while maintaining lasting service lives of the membrane. The gas bubbles which are formed roll along on the lower edge of the electrode because of the inclination of the electrode bars with respect to the horizontal, collide with bubbles still adhering to the edge of the electrode and coalesce. This in turn leads to the gas bubbles being accelerated on account of the increasing volume, that is to say the effect accelerates itself. At the same time, the volume of gas in the electroactive zone decreases, as a result of which a lower cell voltage is reached. A suction effect, which is caused by the movement of the gas bubbles along the edge of the electrode, ensures that fresh electrolyte is sucked into the electroactive zone between membrane or diaphragm and electrode, which for example in chloralkali electrolysis is a necessary precondition for a long membrane service life. Furthermore, directed flow occurs, since all the gas bubbles are forced in one direction. As a result, the density of the electrolyte/gas mixture decreases on one side because of the increasing gas content, which leads to an internal circulation which, compared with the occurrence in the electrolyte stream, is greater by the factor 10 to 100. This achieves excellent homogenization of the electrolyte.
It has been shown to be particularly advantageous for the angle of inclination of the venetian-blind apertures with respect to the horizontal to be between 7xc2x0 and 10xc2x0.
In a refinement which is particularly preferred in constructional terms, provision is made for the underside of the respective housing to be arranged parallel to the horizontal and for the venetian-blind apertures of the anode and cathode to be arranged at an angle with respect to the underside of the respective housing. The electrolysis apparatus per se then has to be modified only slightly with respect to known electrolysis apparatus; it is merely necessary for the anode and the cathode to be installed at an angle and configured appropriately at the edge in order that they can be installed appropriately.
Alternatively, provision can also be made for the underside of the respective housing to be arranged at an angle with respect to the horizontal. The individual housings then have to be changed very little by comparison with previously known housings, having merely to be installed at an angle with respect to the horizontal, as a result of which the venetian-blind apertures of cathode and anode are automatically arranged at an angle with respect to the horizontal.